[llvm] [RISCV][WIP] Let RA do the CSR saves. (PR #90819)

[Craig Topper via llvm-commits]

topperc wrote:

I fought some old post about improving shrink wrapping. https://github.com/sifive/riscv-llvm-internal/pull/1932 not sure how much of it was implemented.

CC @francisvm 

https://github.com/llvm/llvm-project/pull/90819